Assisted Mechanical Ventilation and Its Effect on End-Tidal Carbon Dioxide Levels in Anesthetized California Sea Lions
Anesthesia in many marine mammal species may be complicated by anatomic and physiologic adaptations to a marine existence. A highly developed diving reflex that includes bradycardia and prolonged apnea may be responsible for anesthetic complications reported in the past. Of special interest is an apparent tolerance for increasing carbon dioxide levels without stimulation of respiratory centers that may lead to a potentially dangerous acidosis. Continuous monitoring of end-tidal carbon dioxide (EtCO2) levels may bring new insight into the causes of marine mammal anesthetic complications. In addition, the physiologic effects and potential benefits of mechanically assisted ventilation have not been well described in marine mammals.
Thirty-five stranded California sea lions (Zalophus californianus) were anesthetized for a variety of clinical procedures at a rehabilitation center. The animals were given 0.02 mg/kg of atropine IM and, approximately 10 minutes later, 70 µg/kg of medetomidine with 1 mg/kg of a 1:1 solution of zolazepam and tiletamine IM. All animals were intubated, given 100% oxygen, and maintained with isoflurane. During the procedure, isoflurane level, anesthetic plane, heart rate, respiratory rate, esophageal temperature, oxygen saturation, and EtCO2 levels were monitored and recorded approximately every 5 minutes. Blood gas parameters were also recorded in some individuals.
Eighteen sea lions were anesthetized without providing assisted ventilation. Twelve animals were provided with mechanically assisted ventilation at a rate of 6–8 breaths per minute (bpm) and a volume of approximately 20 ml/kg/breath. Mechanically assisted ventilation was given to an additional five sea lions at the same volume but at a rate of 10–12 bpm. The ventilation volume of 20 ml/kg/breath was found to produce an inspiratory pressure of less than 20 cm H2O in most animals. If the resulting inspiratory pressure was greater than 25 cm H2O, the ventilation volume was decreased.
The EtCO2 levels for animals with no assisted ventilation ranged from 26 to 80 mm Hg with a mean ± SD of 51±10 mm Hg. The EtCO2 levels in animals assisted at a rate of 6–8 bpm ranged from 20 to 65 mm Hg with a mean ± SD of 47±10 mm Hg. In the sea lions provided with assisted ventilation at a rate of 10–12 bpm, EtCO2 levels ranged from 32 to 53 mm Hg and had a mean±SD of 42±6. The mean EtCO2 levels that were recorded from animals receiving assisted ventilation at a rate of 10–12 bpm were significantly lower than those recorded from the other two groups of sea lions. Blood pH levels of greater than 7.5 were recorded in two animals that were ventilated at the higher rate and was likely indicative of alkalosis.
Mechanically assisted ventilation resulted in lower mean EtCO2 levels in anesthetized California sea lions and appeared to decrease the occurrence of very high levels (greater than 60 mm Hg). It is likely that mechanically assisted ventilation is not required for all anesthetized sea lions but should be considered for procedures of a relatively long duration, for animals that must be a maintained in a position that may interfere with normal thoracic expansion, or for those animals in which rising EtCO2 levels are observed. For most sea lions, a volume of 20 ml/kg/breath at a rate of 10–12 bpm is recommended initially, but should be adjusted for each individual. Measurement of blood gas parameters is also recommended due to the possibility of developing an alkalosis in sea lions as a result of mechanical ventilation.